Controlled-source electromagnetic (“CSEM”) surveys are an important geophysical tool for evaluating the presence of hydrocarbon-bearing strata within the earth. CSEM surveys typically record the electromagnetic signal induced in the earth by a source (transmitter) and measured at one or more receivers. The behavior of this signal as a function of transmitter signal frequency and separation (called offset) between transmitter and receiver can be diagnostic of rock properties associated with the presence or absence of hydrocarbons. Specifically, CSEM measurements are used to determine the spatially varying resistivity of the subsurface, and resistivity anomalies enable predictions to be made about the hydrocarbon potential of the subsurface region.
In the marine environment, CSEM data are typically acquired by towing an electric dipole transmitter antenna 11 among a number of receivers 12 positioned on the seafloor 13 (FIG. 1). The receivers are directional and usually have multiple sensors designed to record different vector components of the electric and/or magnetic fields. In typical applications, the receivers will have a minimum of two horizontal electric-field antennae. In addition, the receivers may have a vertical electric-field antenna and/or horizontal and vertical magnetic-field antennae. Alternative configurations include stationary transmitters on the seafloor or in the water column as well as magnetic transmitter antennae. “Offline” or “broadside” data refer to data acquired with the receiver displaced laterally from the tow line.
While alternative, towed configurations are known (see for example U.S. Pat. No. 4,617,518 to Srnka), the receivers most commonly used in CSEM surveys sink freely to the seafloor during the deployment. Knowledge of the actual orientation of the electromagnetic detectors on the seafloor is critical for proper interpretation and imaging of the CSEM data. Two types of approaches have been used to estimate these final orientations. One approach is to include a measurement system on the receivers, but these do not typically provide reliable information about the orientation of the receiver's electric and magnetic antennae. The other approach is processing-based techniques that use simplifying assumptions and provide only approximations to the receiver orientations. Receiver orientations have been previously analyzed by one or the other of these two approaches or combinations of both. Examples of each approach include:
Compass measurements: electronic or locking compasses installed on the receiver are used to measure the local direction of the earth's static magnetic field relative to the receiver antennae. These measurements are recorded and become accessible when the receiver is recovered after the survey is finished. (Key, et al., “Mapping 3D salt using the 2D marine magnetotelluric method: Case study from Gemini Prospect, Gulf of Mexico,” Geophysics 71, B17-B27 (2006)). The inclination and declination of the static field are routinely mapped and published (http://www.ndgc.noaa.gov/geomag/geomag.shtml). PCT Patent Application Publication WO 2007/136451 (Summerfield and Phillips) further refines receiver orientation measurements based on attitude sensors (such as compasses) by measuring the deviation of the electric and magnetic sensor positions from their nominal design positions relative to the receiver body.
Polarization analysis: See Constable and Cox, “Marine controlled-source electromagnetic sounding 2. The PEGASUS experiment,” Jour. Geophys. Res. 101, 5519-5530 (1996); and Behrens, “The Detection of Electrical Anisotropy in 35 Ma Pacific Lithosphere: Results from a marine controlled-source electromagnetic survey and implications for hydration of the upper mantle,” University of California thesis (2005). In a one-dimensional earth, the strongest horizontal electric field is parallel to the towed electric dipole source (inline electric field) while the strongest horizontal magnetic field is perpendicular to the towed source (crossline magnetic field). Maximizing the energy in these components gives an estimate of the receiver orientation relative to the tow line orientation. Mittet et al. (PCT Patent Application Publication WO 2008/032065; also “On the orientation and absolute phase of marine CSEM receivers,” Geophysics 72, F145-F155 (2007)) also describe the polarization analysis method with the minor additions of weighting the electromagnetic data in the least-squares analysis and median filtering the predicted orientations for different source and receiver offsets.
Magnetotelluric data coherency and correlation between two receivers: This method (see Behrens, op. cit.) determines the relative rotation angle between two receivers using the background electromagnetic signals generated by the interaction of the solar wind and the ionosphere. Generalizations of this method (Egbert, “Robust multiple station magnetotelluric data processing,” Geophys. J. Int. 130, 475-496 (1997)) improve upon receiver-by-receiver orientation analysis to determine best estimates for the orientations of a group of receivers.
Inversion: See Mittet et al., E020, “Inversion of SBL data acquired in shallow waters,” EAGE 66th Conference & Exhibition—Paris, France, Jun. 7-10 (2004); and Lu, PCT Patent Application Publication WO2007/018810. Receiver azimuth and tilts are determined by inversion of measured EM data, either simultaneously with inversion for subsurface resistivity or based on a fixed resistivity model.
All of these methods have limitations. Compass measurements are subject to stray magnetic fields in the receivers and local errors in the static field measurements, and are not accurate enough for practical applications in subsurface hydrocarbon detections.
Polarization analysis requires that at least one towline must pass close to the receiver (online data). When used with online data, polarization analysis is a relatively robust method for extracting the maximum inline component of the electric field or crossline component of the magnetic field because these components depend on the cosine of the error in receiver orientation. For angular errors in the range of 5 to 10 degrees typical of polarization analysis, the cosine of the error will be off by less than 2%. Other data components, such as the crossline component of broadside data, will depend on the sine of the angular error, so that a 10 degree error will have a 17% impact on these components. Polarization analysis alone is therefore unsuitable to predict these more sensitive components. Orientations determined by this technique are furthermore subject to data limitation caused by receiver saturation (signals too large to be digitized), feathering of the CSEM source antenna, and breakdown of the one-dimensional earth approximation. Polarization analysis does not generalize to three dimensions to determine the tilt of vertical receiver antennae.
In order to find the receiver azimuth, magnetotelluric coherency requires the azimuth of the reference receiver be known. Success in using this method is dependent on whether high quality natural signals are recorded by both receivers. This method is even more strongly influenced by three-dimensional variation of the subsurface and is typically less accurate than polarization analysis. Like polarization analysis, magnetotelluric coherency does not generalize to determine the tilt of vertical receiver antennae;
Inversion can provide accurate results if the model of the earth resistivity is close enough to the reality. It is, however, computationally intensive because multiple solutions of the forward modeling problem (i.e., solving Maxwell's equations by numerical methods) are required and sensitive to errors introduced by local minima in the objective function from both the variation of receiver orientations and the subsurface resistivities.
Thus, an improved method for determining receiver orientation is needed, and the present invention satisfies this need.